US Population Pyramid Question

I noticed a discrepancy in US Population figures that I can’t quite explain:

  1. The # of births was** increasing from 1976-1990 and also 1997-2007** (let’s call them Early Millennials and Late Millennials):

You can also check this on Page 3 of this CDC PDF:

  1. However, US Population Pyramids show the population **under 20 declining **as of 2015/2016/2017:


Why? The Early Millennials make an appearance in the population pyramid, but the Late Millennials do not. It’s as if there was no baby boom from 1997-2007, but we know that there was, based on the # of births above.

Any thoughts on this discrepancy?

Don’t forget that the population pyramid also includes immigration. The majority of immigrants likely arrive without a lot of under-20 children, as a guess. For the USA, immigration is a significant factor.